The present invention relates to calibration of transmit and receive paths of an adaptive array antenna comprising multiple antenna branches in a radio base station arrangement.
Adaptive antennas have shown impressive potential for increasing capacity in cellular networks. Extensive theoretical works as well as field trials confirm this statement. When utilizing adaptive antenna beams, it is desired to steer the beams of the antenna continuously, and at the same time to steer nulls independently. Steering is accomplished by adjusting the relative phase and amplitude between the antenna branches in such a way that constructive or destructive interference will occur for beam pointing and nulling, respectively. That is, as opposed to a switched beam implementation where the best beam is chosen, such a solution deals with the actual implementation of continuous tracking and active disturbance nulling. This calls for a precise phase and amplitude adjustment of all involved hardware and software that constitute the signal path from the xe2x80x9csourcexe2x80x9d to the antenna connector.
Today, capacity in a cellular network is limited by the ability to withstand co-channel interference. A certain xe2x80x9cguardxe2x80x9d level of the unwanted signal power received by the mobile has to be maintained below the desired signal power. This is assured by keeping base stations transmitting on the same frequency at a certain minimum geometrical distance from each other. In this way, other base stations transmitting on the same frequency will not disturb each other""s mobiles. Base stations transmitting in other frequency groups may then on the other hand be placed in an interleaved fashion between these first mentioned base stations. This approach may be repeated until all the available frequencies are occupied, but in such a manner that they do not interfere with each other. The distances between the base stations and the total available number of frequencies result in a figure of merit for the capacity of a cellular network.
It is evident that if the power from interfering base stations as received by mobiles (and also received power at the base station) could be reduced in another way than physically relocating base stations, the capacity could be increased in the network. Frequencies could then be reused more often in a given physical area, in this way serving more subscribers. Limiting the transmitted power can be achieved by means of an antenna that has a narrower opening angle. In addition to this, the antenna beam has to be able to continuously track the mobile within its operating sector. Mainly two implementations exist: the switched beam solution and the full adaptive solution. In both cases an array of antenna elements is used, but in the latter case each element may be individually controlled, whereas in the former case only one out of several beams may be chosen. In the former case, a set of pre-set antenna beams is used whereas in the latter, one single beam is used, but continuously being tracked towards the mobile. This we refer to as a xe2x80x9cfull adaptive antennaxe2x80x9d in this document.
A necessary condition is that each antenna path is calibrated so that possible irregularities may be accounted for and compensated. Both uplink and downlink has normally to be calibrated. However, the problem is more pronounced in the downlink direction (base station-to-mobile) whereas in the uplink (mobile-to-base station) the beam-forming algorithms can be made to auto-correct for phase errors.
The mostly used implementation of adaptive antennas is to use switched beams. Then the beam-former is preferably placed at the top of the mast close to the antenna array, and does not constitute any large phase errors. One connector at the input of the beam-former gives the proper excitation to all the antenna elements at the same time. This gives a beam in a certain direction. Choosing another input connector gives a beam in yet another direction. So, depending on for example the strongest received signal, the proper beam port is chosen and in this way no calibration is needed since it always only means one single RF path to the antenna. However, the above switched beam implementation does not give the freedom of steering the beam to an arbitrary position, nor the nulling out of interfering signals.
The most obvious solution for calibrating an antenna system would be to insert a signal at an appropriate position (in the transmit path) and then tracking the signal as it propagates through the system. The signal has to be detected at some close point to the antenna and then compared to the inserted signal. At first glance it needs additional hardware and detectors to be implemented. Usually an external source is applied to the equipment, and input to output signals are then compared. The difference between the two will then give the necessary phase and amplitude to be compensated for. The procedure in the receiving direction would be similar as for the transmit case. It is evident that the approach needs additional generator and detector equipment and it is difficult to perform such a calibration when the base station is under operation.
Usually calibration is made on the lab bench prior to installation, or by calibrating each part of the RF chain separately. Calibration of an array antenna only in the factory is not satisfactory with respect to usage at a site. Calibration has to be made during operation in some particular manner, and continuously being monitored. This is normally not fully supported today. If installed, one way would be to use an external separate generator to measure the signal path both in transmit branches and receive branches. This requires additional receivers tuned to the proper frequency together with some software implementation to calculate the amplitude and phase offsets. Moreover, signal insertion points and extraction points must be implemented into the base station.
Consequently calibration of array antennas is a problem without a proper solution today. Therefore there is a need for a method to calibrate array antennas at the site with a minimum of hardware requirement and which applies to base stations having a xe2x80x9cfull adaptive antennaxe2x80x9d implementation.
The present disclosure proposes a method and a system for achieving calibration with a minimum of hardware implementations and applies to base stations that have a xe2x80x9cfull adaptive antennaxe2x80x9d implementation. It accounts for irregularities from the source generator through channelizers, up-converters, mixers, power-amplifiers, and filters. A proposed illustrative embodiment of the set-up and routine accomplishes both transmit path and receive path calibration. The arrangement according to the invention uses the internal base station radio parts by sequentially switching signal paths from transmitter to receiver and thereby not needing external equipment. The necessary switches then are combined with the duplex filters preferably close to the base station. The method can simply be visualized by a basic flow diagram presenting the three main steps being a first step of receive calibration to be repeated for all available receive frequencies, a second step of pre-transmit calibration and a third step of transmit calibration to be repeated for all available transmit frequencies.
A method according to the present invention is set forth by the independent claim 1 and the dependent claims 2-4. A system for calibration of an adaptable array antenna is set forth by the independent claim 5 and further embodiment are set forth by the dependent claims 6 to 9.